Network Working Group D. Moberg
Request for Comments: 4130 Cyclone Commerce
Category: Standards Track R. Drummond
Drummond Group Inc.
July 2005
MIME-Based Secure Peer-to-PeerBusiness Data Interchange Using HTTP,Applicability Statement 2 (AS2)
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document provides an applicability statement (RFC 2026, Section 3.2) that describes how to exchange structured business data securely
using the HTTP transfer protocol, instead of SMTP; the applicability
statement for SMTP is found in RFC 3335. Structured business data
may be XML; Electronic Data Interchange (EDI) in either the American
National Standards Committee (ANSI) X12 format or the UN Electronic
Data Interchange for Administration, Commerce, and Transport
(UN/EDIFACT) format; or other structured data formats. The data is
packaged using standard MIME structures. Authentication and data
confidentiality are obtained by using Cryptographic Message Syntax
with S/MIME security body parts. Authenticated acknowledgements make
use of multipart/signed Message Disposition Notification (MDN)
responses to the original HTTP message. This applicability statement
is informally referred to as "AS2" because it is the second
applicability statement, produced after "AS1", RFC 3335.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Receipt: The functional message that is sent from a receiver to a
sender to acknowledge receipt of an EDI/EC interchange.
This message may be either synchronous or asynchronous in
nature.
Signed Receipt: A receipt with a digital signature.
Synchronous Receipt: A receipt returned to the sender during the same
HTTP session as the sender's original message.
Asynchronous Receipt: A receipt returned to the sender on a different
communication session than the sender's original message
session.
Message Disposition Notification (MDN): The Internet messaging format
used to convey a receipt. This term is used interchangeably
with receipt. A MDN is a receipt.
Non-repudiation of receipt (NRR): A "legal event" that occurs when
the original sender of an signed EDI/EC interchange has
verified the signed receipt coming back from the receiver.
The receipt contains data identifying the original message
for which it is a receipt, including the message-ID and a
cryptographic hash (MIC). The original sender must retain
suitable records providing evidence concerning the message
content, its message-ID, and its hash value. The original
sender verifies that the retained hash value is the same as
the digest of the original message, as reported in the
signed receipt. NRR is not considered a technical message,
but instead is thought of as an outcome of possessing
relevant evidence.
S/MIME: A format and protocol for adding cryptographic signature
and/or encryption services to Internet MIME messages.
Cryptographic Message Syntax (CMS): An encapsulation syntax used to
digitally sign, digest, authenticate, or encrypt arbitrary
messages.
SHA-1: A secure, one-way hash algorithm used in conjunction with
digital signature. This is the recommended algorithm for
AS2.
MD5: A secure, one-way hash algorithm used in conjunction with
digital signature. This algorithm is allowed in AS2.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
MIC: The message integrity check (MIC), also called the message
digest, is the digest output of the hash algorithm used by
the digital signature. The digital signature is computed
over the MIC.
User Agent (UA): The application that handles and processes the AS2
request.
2. Overview2.1. Overall Operation
A HTTP POST operation [3] is used to send appropriately packaged EDI,
XML, or other business data. The Request-URI ([3], Section 9.5)
identifies a process for unpacking and handling the message data and
for generating a reply for the client that contains a message
disposition acknowledgement (MDN), either signed or unsigned. The
MDN is either returned in the HTTP response message body or by a new
HTTP POST operation to a URL for the original sender.
This request/reply transactional interchange can provide secure,
reliable, and authenticated transport for EDI or other business data
using HTTP as a transfer protocol.
The security protocols and structures used also support auditable
records of these document data transmissions, acknowledgements, and
authentication.
2.2. Purpose of a Security Guideline for MIME EDI
The purpose of these specifications is to ensure interoperability
between B2B EC user agents, invoking some or all of the commonly
expected security features. This document is also NOT limited to
strict EDI use; it applies to any electronic commerce application for
which business data needs to be exchanged over the Internet in a
secure manner.
2.3. Definitions2.3.1. The Secure Transmission Loop
This document's focus is on the formats and protocols for exchanging
EDI/EC content securely in the Internet's HTTP environment.
In the "secure transmission loop" for EDI/EC, one organization sends
a signed and encrypted EDI/EC interchange to another organization and
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
requests a signed receipt, and later the receiving organization sends
this signed receipt back to the sending organization. In other
words, the following transpires:
o The organization sending EDI/EC data signs and encrypts the
data using S/MIME. In addition, the message will request that
a signed receipt be returned to the sender. To support NRR,
the original sender retains records of the message, message-ID,
and digest (MIC) value.
o The receiving organization decrypts the message and verifies
the signature, resulting in verified integrity of the data and
authenticity of the sender.
o The receiving organization then returns a signed receipt using
the HTTP reply body or a separate HTTP POST operation to the
sending organization in the form of a signed message
disposition notification. This signed receipt will contain the
hash of the received message, allowing the original sender to
have evidence that the received message was authenticated
and/or decrypted properly by the receiver.
The above describes functionality that, if implemented, will satisfy
all security requirements and implement non-repudiation of receipt
for the exchange. This specification, however, leaves full
flexibility for users to decide the degree to which they want to
deploy those security features with their trading partners.
2.3.2. Definition of Receipts
The term used for both the functional activity and the message for
acknowledging delivery of an EDI/EC interchange is "receipt" or
"signed receipt". The first term is used if the acknowledgment is
for an interchange resulting in a receipt that is NOT signed. The
second term is used if the acknowledgement is for an interchange
resulting in a receipt that IS signed.
The term non-repudiation of receipt (NRR) is often used in
combination with receipts. NRR refers to a legal event that occurs
only when the original sender of an interchange has verified the
signed receipt coming back from recipient of the message, and has
verified that the returned MIC value inside the MDN matches the
previously recorded value for the original message.
NRR is best established when both the original message and the
receipt make use of digital signatures. See the Security
Considerations section for some cautions regarding NRR.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
For information on how to format and process receipts in AS2, refer
to Section 7.
2.4. Assumptions2.4.1. EDI/EC Process Assumptions
o Encrypted object is an EDI/EC Interchange.
This specification assumes that a typical EDI/EC interchange is the
lowest-level object that will be subject to security services.
Specifically, in EDI ANSI X12, this means that anything between and
including, segments ISA and IEA is secured. In EDIFACT, this means
that anything between, and including, segments UNA/UNB and UNZ is
secured. In other words, the EDI/EC interchanges including envelope
segments remain intact and unreadable during fully secured transport.
o EDI envelope headers are encrypted.
Congruent with the above statement, EDI envelope headers are NOT
visible in the MIME package.
In order to optimize routing from existing commercial EDI networks
(called Value Added Networks or VANs) to the Internet, it would be
useful to make some envelope information visible. This
specification, however, provides no support for this optimization.
o X12.58 and UN/EDIFACT Security Considerations
The most common EDI standards bodies, ANSI X12 and EDIFACT, have
defined internal provisions for security. X12.58 is the security
mechanism for ANSI X12, and AUTACK provides security for EDIFACT.
This specification does NOT dictate use or non-use of these security
standards. They are both fully compatible, though possibly
redundant, with this specification.
2.4.2. Flexibility Assumptions
o Encrypted or Unencrypted Data
This specification allows for EDI/EC message exchange in which the
EDI/EC data can be either unprotected or protected by means of
encryption.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
o Signed or Unsigned Data
This specification allows for EDI/EC message exchange with or without
digital signature of the original EDI transmission.
o Optional Use of Receipt
This specification allows for EDI/EC message transmission with or
without a request for receipt notification. A signed receipt
notification is requested; however, a MIC value is REQUIRED as part
of the returned receipt, except when a severe error condition
prevents computation of the digest value. In the exceptional case, a
signed receipt should be returned with an error message that
effectively explains why the MIC is absent.
o Use of Synchronous or Asynchronous Receipts
In addition to a receipt request, this specification allows the
specification of the type of receipt that should be returned. It
supports synchronous or asynchronous receipts in the MDN format
specified in Section 7 of this document.
o Security Formatting
This specification relies on the guidelines set forth in RFC3851/3852 [7] "S/MIME Version 3.1 Message Specification;
Cryptographic Message Syntax".
o Hash Function, Message Digest Choices
When a signature is used, it is RECOMMENDED that the SHA-1 hash
algorithm be used for all outgoing messages, and that both MD5 and
SHA-1 be supported for incoming messages.
o Permutation Summary
In summary, the following twelve security permutations are possible
in any given trading relationship:
1. Sender sends un-encrypted data and does NOT request a receipt.
2. Sender sends un-encrypted data and requests an unsigned receipt.
Receiver sends back the unsigned receipt.
3. Sender sends un-encrypted data and requests a signed receipt.
Receiver sends back the signed receipt.
4. Sender sends encrypted data and does NOT request a receipt.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
5. Sender sends encrypted data and requests an unsigned receipt.
Receiver sends back the unsigned receipt.
6. Sender sends encrypted data and requests a signed receipt.
Receiver sends back the signed receipt.
7. Sender sends signed data and does NOT request a signed or
unsigned receipt.
8. Sender sends signed data and requests an unsigned receipt.
Receiver sends back the unsigned receipt.
9. Sender sends signed data and requests a signed receipt.
Receiver sends back the signed receipt.
10. Sender sends encrypted and signed data and does NOT request a
signed or unsigned receipt.
11. Sender sends encrypted and signed data and requests an unsigned
receipt. Receiver sends back the unsigned receipt.
12. Sender sends encrypted and signed data and requests a signed
receipt. Receiver sends back the signed receipt.
Users can choose any of the twelve possibilities, but only the last
example (12), when a signed receipt is requested, offers the whole
suite of security features described in Section 2.3.1, "The Secure
Transmission Loop".
Additionally, the receipts discussed above may be either synchronous
or asynchronous depending on the type requested. The use of either
the synchronous or asynchronous receipts does not change the nature
of the secure transmission loop in support of NRR.
3. Referenced RFCs and Their Contributions3.1. RFC 2616 HTTP v1.1 [3]
This document specifies how data is transferred using HTTP.
3.2. RFC 1847 MIME Security Multiparts [6]
This document defines security multipart for MIME:
multipart/encrypted and multipart/signed.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 20053.3. RFC 3462 Multipart/Report [8]
This RFC defines the use of the multipart/report content type,
something that the MDN RFC 3798 builds upon.
3.4. RFC 1767 EDI Content [2]
This RFC defines the use of content type "application" for ANSI X12
(application/EDI-X12), EDIFACT (application/EDIFACT), and mutually
defined EDI (application/EDI-Consent).
3.5. RFC 2045, 2046, and 2049 MIME [1]
These are the basic MIME standards, upon which all MIME related RFCs
build, including this one. Key contributions include definitions of
"content type", "sub-type", and "multipart", as well as encoding
guidelines, which establish 7-bit US-ASCII as the canonical character
set to be used in Internet messaging.
3.6. RFC 3798 Message Disposition Notification [5]
This Internet RFC defines how an MDN is requested, and the format and
syntax of the MDN. The MDN is the basis upon which receipts and
signed receipts are defined in this specification.
3.7. RFC 3851 and 3852 S/MIME Version 3.1 Message Specifications and Cryptographic Message Syntax (CMS) [7]
This specification describes how S/MIME will carry CMS Objects.
3.8. RFC 3023 XML Media Types [10]
This RFC defines the use of content type "application" for XML
(application/xml).
4. Structure of an AS2 Message4.1. Introduction
The basic structure of an AS2 message consists of MIME format inside
an HTTP message with a few additional specific AS2 headers. The
structures below are described hierarchically in terms of which RFCs
are applied to form the specific structure. For details of how to
code in compliance with all RFCs involved, turn directly to the RFCs
referenced. Any difference between AS2 implantations and RFCs are
mentioned specifically in the sections below.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Content-Type: application/EDIFACT
Content-Type: application/edi-consent
Content-Type: application/XML
5. HTTP Considerations5.1. Sending EDI in HTTP POST Requests
The request line will have the form: "POST Request-URI HTTP/1.1",
with spaces and followed by a CRLF. The Request URI is typically
exchanged out of band, as part of setting up a bilateral trading
partner agreement. Applications SHOULD be prepared to deal with an
initial reply containing a status indicating a need for
authentication of the usual types used for authorizing access to the
Request-URI ([3], Section 10.4.2 and elsewhere).
The request line is followed by entity headers specifying content
length ([3], Section 14.14) and content type ([3], Section 14.18).
The Host request header ([3], Sections 9 and 14.23) is also included.
When using Transport Layer Security [15] or SSLv3, the request-URI
SHOULD indicate the appropriate scheme value, HTTPS. Usually only a
multipart/signed message body would be sent using TLS, as encrypted
message bodies would be redundant. However, encrypted message bodies
are not prohibited.
The receiving AS2 system MAY disconnect from the sending AS2 system
before completing the reception of the entire entity if it determines
that the entity being sent is too large to process.
For HTTP version 1.1, TCP persistent connections are the default,
([3] Sections 8.1.2, 8.2, and 19.7.1). A number of other differences
exist because HTTP does not conform to MIME [1] as used in SMTP
transport. Relevant differences are summarized below.
5.2. Unused MIME Headers and Operations5.2.1. Content-Transfer-Encoding Not Used in HTTP Transport
HTTP can handle binary data and so there is no need to use the
content transfer encodings of MIME [1]. This difference is discussed
in [3], Section 19.4.5. However, a content transfer encoding value
of binary or 8-bit is permissible but not required. The absence of
this header MUST NOT result in transaction failure. Content transfer
encoding of MIME bodyparts within the AS2 message body is also
allowed.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 20055.2.2. Message Bodies
In [3], Section 3.7.2, it is explicitly noted that multiparts MUST
have null epilogues.
In [4], Section 5.4.1, options for large file processing are
discussed for SMTP transport. For HTTP, large files SHOULD be
handled correctly by the TCP layer. However, in [3], Sections 3.5
and 3.6 discuss some options for compressing or chunking entities to
be transferred. In [3], Section 8.1.2.2 discusses a pipelining
option that is useful for segmenting large amounts of data.
5.3. Modification of MIME or Other Headers or Parameters Used5.3.1. Content-Length
The use of the content-length header MUST follow the guidelines of
[3], specifically Sections 4.4 and 14.13.
5.3.2. Final Recipient and Original Recipient
The final and original recipient values SHOULD be the same value.
These values MUST NOT be aliases or mailing lists.
5.3.3. Message-Id and Original-Message-Id
Message-Id and Original-Message-Id is formatted as defined in RFC2822 [9]:
"<" id-left "@" id-right ">" (RFC 2822, 3.6.4)
Message-Id length is a maximum of 998 characters. For maximum
backward compatibility, Message-Id length SHOULD be 255 characters or
less. Message-Id SHOULD be globally unique, and id-right SHOULD be
something unique to the sending host environment (e.g., a host name).
When sending a message, always include the angle brackets. Angle
brackets are not part of the Message-Id value. For maximum backward
compatibility, when receiving a message, do not check for angle
brackets. When creating the Original-Message-Id header in an MDN,
always use the exact syntax as received on the original message;
don't strip or add angle brackets.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 20055.3.4. Host Header
The host request header field MUST be included in the POST request
made when sending business data. This field is intended to allow one
server IP address to service multiple hostnames, and potentially to
conserve IP addresses. See [3], Sections 14.23 and 19.5.1.
5.4. HTTP Response Status Codes
The status codes return status concerning HTTP operations. For
example, the status code 401, together with the WWW-Authenticate
header, is used to challenge the client to repeat the request with an
Authorization header. Other explicit status codes are documented in
[3], Section 6.1.1 and throughout Section 10.
For errors in the request-URI, 400 ("Bad Request"), 404 ("Not
Found"), and similar codes are appropriate status codes. These codes
and their semantics are specified by [3]. A careful examination of
these codes and their semantics should be made before implementing
any retry functionality. Retries SHOULD NOT be made if the error is
not transient or if retries are explicitly discouraged.
5.5. HTTP Error Recovery
If the HTTP client fails to read the HTTP server response data, the
POST operation with identical content, including same Message-ID,
SHOULD be repeated, if the condition is transient.
The Message-ID on a POST operation can be reused if and only if all
of the content (including the original Date) is identical.
Details of the retry process (including time intervals to pause,
number of retries to attempt, and timeouts for retrying) are
implementation dependent. These settings are selected as part of the
trading partner agreement.
Servers SHOULD be prepared to receive a POST with a repeated
Message-ID. The MIME reply body previously sent SHOULD be resent,
including the MDN and other MIME parts.
6. Additional AS2-Specific HTTP Headers
The following headers are to be included in all AS2 messages and all
AS2 MDNs, except for asynchronous MDNs that are sent using SMTP and
that follow the AS1 semantics[4].
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 20056.1. AS2 Version Header
To promote backward compatibility, AS2 includes a version header:
AS2-Version: 1.0 - Used in all implementations of this
specification. 1.x will be interpreted as 1.0 by
all implementations with the "AS2 Version: 1.0"
header. That is, only the most significant digit
is used as the version identifier for those not
implementing additional non-AS2-specified
functionality. "AS2-Version: 1.0 through 1.9" MAY
be used. All implementations MUST interpret "1.0
through 1.9" as implementing this specification.
However, an implementation MAY extend this
specification with additional functionality by
specifying versions 1.1 through 1.9. If this
mechanism is used, the additional functionality
MUST be completely transparent to implementations
with the "AS2-Version: 1.0" designation.
AS2-Version: 1.1 - Designates those implementations that support
compression as defined by RFC 3274.
Receiving systems MUST NOT fail due to the absence of the AS2-Version
header. Its absence would indicate that the message is from an
implementation based on a previous version of this specification.
6.2. AS2 System Identifiers
To aid the receiving system in identifying the sending system,
AS2-From and AS2-To headers are used.
AS2-From: < AS2-name >
AS2-To: < AS2-name >
These AS2 headers contain textual values, as described below,
identifying the sender/receiver of a data exchange. Their values may
be company specific, such as Data Universal Numbering System (DUNS)
numbers, or they may be simply identification strings agreed upon
between the trading partners.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
AS2-text = "!" / ; printable ASCII characters
%d35-91 / ; except double-quote (%d34)
%d93-126 ; or backslash (%d92)
AS2-qtext = AS2-text / SP ; allow space only in quoted text
AS2-quoted-pair = "\" DQUOTE / ; \" or
"\" "\" ; \\
AS2-quoted-name = DQUOTE 1*128( AS2-qtext /
AS2-quoted-pair) DQUOTE
AS2-atomic-name = 1*128AS2-text
AS2-name = AS2-atomic-name / AS2-quoted-name
The AS2-From header value and the AS2-To header value MUST each be an
AS2-name, MUST each be comprised of from 1 to 128 printable ASCII
characters, and MUST NOT be folded. The value in each of these
headers is case-sensitive. The string definitions given above are in
ABNF format [14].
The AS2-quoted-name SHOULD be used only if the AS2-name does not
conform to AS2-atomic-name.
The AS2-To and AS2-From header fields MUST be present in all AS2
messages and AS2 MDNs whether asynchronous or synchronous in nature,
except for asynchronous MDNs, which are sent using SMTP.
The AS2-name for the AS2-To header in a response or MDN MUST match
the AS2-name of the AS2-From header in the corresponding request
message. Likewise, the AS2-name for the AS2-From header in a
response or MDN MUST match the AS2-name of the AS2-To header in the
corresponding AS2 request message.
The sending system may choose to limit the possible AS2-To/AS2-From
textual values but MUST not exceed them. The receiving system MUST
make no restrictions on the textual values and SHOULD handle all
possible implementations. However, implementers must be aware that
older AS2 products may not adhere to this convention. Trading
partner agreements should be made to ensure that older products can
support the system identifiers that are used.
There is no required response to a client request containing invalid
or unknown AS2-From or AS2-To header values. The receiving AS2
system MAY return an unsigned MDN with an explanation of the error,
if the sending system requested an MDN.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 20057. Structure and Processing of an MDN Message7.1. Introduction
In order to support non-repudiation of receipt, a signed receipt,
based on digitally signing a message disposition notification, is to
be implemented by a receiving trading partner's UA. The message
disposition notification, specified by RFC 3798, is digitally signed
by a receiving trading partner as part of a multipart/signed MIME
message.
The following support for signed receipts is REQUIRED:
1. The ability to create a multipart/report; where the
report-type = disposition-notification.
2. The ability to calculate a message integrity check (MIC) on the
received message. The calculated MIC value will be returned to
the sender of the message inside the signed receipt.
3. The ability to create a multipart/signed content with the
message disposition notification as the first body part, and
the signature as the second body part.
4. The ability to return the signed receipt to the sending trading
partner.
5. The ability to return either a synchronous or an asynchronous
receipt as the sending party requests.
The signed receipt is used to notify a sending trading partner that
requested the signed receipt that:
1. The receiving trading partner acknowledges receipt of the sent
EC Interchange.
2. If the sent message was signed, then the receiving trading
partner has authenticated the sender of the EC Interchange.
3. If the sent message was signed, then the receiving trading
partner has verified the integrity of the sent EC Interchange.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Regardless of whether the EDI/EC Interchange was sent in S/MIME
format, the receiving trading partner's UA MUST provide the following
basic processing:
1. If the sent EDI/EC Interchange is encrypted, then the encrypted
symmetric key and initialization vector (if applicable) is
decrypted using the receiver's private key.
2. The decrypted symmetric encryption key is then used to decrypt
the EDI/EC Interchange.
3. The receiving trading partner authenticates signatures in a
message using the sender's public key. The authentication
algorithm performs the following:
a. The message integrity check (MIC or Message Digest), is
decrypted using the sender's public key.
b. A MIC on the signed contents (the MIME header and encoded
EDI object, as per RFC 1767) in the message received is
calculated using the same one-way hash function that the
sending trading partner used.
c. The MIC extracted from the message that was sent and the MIC
calculated using the same one-way hash function that the
sending trading partner used are compared for equality.
4. The receiving trading partner formats the MDN and sets the
calculated MIC into the "Received-content-MIC" extension field.
5. The receiving trading partner creates a multipart/signed MIME
message according to RFC 1847.
6. The MDN is the first part of the multipart/signed message, and
the digital signature is created over this MDN, including its
MIME headers.
7. The second part of the multipart/signed message contains the
digital signature. The "protocol" option specified in the
second part of the multipart/signed is as follows:
S/MIME: protocol = "application/pkcs-7-signature"
8. The signature information is formatted according to S/MIME
specifications.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
The EC Interchange and the RFC 1767 MIME EDI content header can
actually be part of a multi-part MIME content-type. When the EDI
Interchange is part of a multi-part MIME content-type, the MIC MUST
be calculated across the entire multi-part content, including the
MIME headers.
The signed MDN, when received by the sender of the EDI Interchange,
can be used by the sender as follows:
o As an acknowledgement that the EDI Interchange sent was
delivered and acknowledged by the receiving trading partner.
The receiver does this by returning the original-message-id
of the sent message in the MDN portion of the signed receipt.
o As an acknowledgement that the integrity of the EDI
Interchange was verified by the receiving trading partner.
The receiver does this by returning the calculated MIC of the
received EC Interchange (and 1767 MIME headers) in the
"Received-content-MIC" field of the signed MDN.
o As an acknowledgement that the receiving trading partner has
authenticated the sender of the EDI Interchange.
o As a non-repudiation of receipt when the signed MDN is
successfully verified by the sender with the receiving
trading partner's public key and the returned MIC value
inside the MDN is the same as the digest of the original
message.
7.2. Synchronous and Asynchronous MDNs
The AS2-MDN exists in two varieties: synchronous and asynchronous.
The synchronous AS2-MDN is sent as an HTTP response to an HTTP POST
or as an HTTPS response to an HTTPS POST. This form of AS2-MDN is
called synchronous because the AS2-MDN is returned to the originator
of the POST on the same TCP/IP connection.
The asynchronous AS2-MDN is sent on a separate HTTP, HTTPS, or SMTP
TCP/IP connection. Logically, the asynchronous AS2-MDN is a response
to an AS2 message. However, at the transfer-protocol layer, assuming
that no HTTP pipelining is utilized, the asynchronous AS2-MDN is
delivered on a unique TCP/IP connection, distinct from that used to
deliver the original AS2 message. When handling an asynchronous
request, the HTTP response MUST be sent back before the MDN is
processed and sent on the separate connection.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
When an asynchronous AS2-MDN is requested by the sender of an AS2
message, the synchronous HTTP or HTTPS response returned to the
sender prior to terminating the connection MUST be a transfer-layer
response indicating the success or failure of the data transfer. The
format of such a synchronous response MAY be the same as that
response returned when no AS2-MDN is requested.
The following diagram illustrates the synchronous versus asynchronous
varieties of AS2-MDN delivery using HTTP:
Synchronous AS2-MDN
[Peer1] ----( connect )----> [Peer2]
[Peer1] -----( send )------> [Peer2] [HTTP Request [AS2-Message]]
[Peer1] <---( receive )----- [Peer2] [HTTP Response [AS2-MDN]]
Asynchronous AS2-MDN
[Peer1] ----( connect )----> [Peer2]
[Peer1] -----( send )------> [Peer2] [HTTP Request [AS2-Message]]
[Peer1] <---( receive )----- [Peer2] [HTTP Response]
[Peer1]*<---( connect )----- [Peer2]
[Peer1] <--- ( send )------- [Peer2] [HTTP Request [AS2-MDN]]
[Peer1] ----( receive )----> [Peer2] [HTTP Response]
* Note: An AS2-MDN may be directed to a host different from that of
the sender of the AS2 message. It may utilize a transfer protocol
different from that used to send the original AS2 message.
The advantage of the synchronous MDN is that it can provide the
sender of the AS2 Message with a verifiable confirmation of message
delivery within a synchronous logic flow. However, if the message is
relatively large, the time required to process this message and to
return an AS2-MDN to the sender on the same TCP/IP connection may
exceed the maximum configured time permitted for an IP connection.
The advantage of the asynchronous MDN is that it provides for the
rapid return of a transfer-layer response from the receiver,
confirming the receipt of data, therefore not requiring that a TCP/IP
connection necessarily remain open for very long. However, this
design requires that the asynchronous AS2-MDN contain enough
information to identify the original message uniquely so that, when
received by the AS2 Message originator, the status of the original
AS2 Message can be properly updated based on the contents of the
AS2-MDN.
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Synchronous or asynchronous HTTP or HTTPS MDNs are handled according
to the requirements of this specification.
However, SMTP MDNs are formatted according to the requirements of RFC3335 [4].
7.3. Requesting a Signed Receipt
Message disposition notifications are requested as per RFC 3798. A
request that the receiving user agent issue a message disposition
notification is made by placing the following header into the message
to be sent:
MDN-request-header = "Disposition-notification-to"
":" mail-address
The following example is for requesting an MDN:
Disposition-notification-to: xxx@example.com
This syntax is a residue of the use of MDNs using SMTP transfer.
Because this specification is adjusting the functionality from SMTP
to HTTP while retaining as much as possible from the [4]
functionality, the mail-address MUST be present. The mail-address
field is specified as an RFC 2822 localpart@domain [addr-spec]
address. However, the address is not used to identify where to
return the MDN. Receiving applications MUST ignore the value and
MUST not complain about RFC 2822 address syntax violations.
When requesting MDN-based receipts, the originator supplies
additional extension headers that precede the message body. These
header "tags" are as follows:
A Message-ID header is added to support message reconciliation, so
that an Original-Message-Id value can be returned in the body part of
MDN. Other headers, especially "Subject" and "Date", SHOULD be
supplied; the values of these headers are often mentioned in the
human-readable section of a MDN to aid in identifying the original
message.
MDNs will be returned in the HTTP response when requested, unless an
asynchronous return is requested.
To request an asynchronous message disposition notification, the
following header is placed into the message that is sent:
Receipt-Delivery-Option: return-URL
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Here is an example requesting that the MDN be asynchronous:
Receipt-Delivery-Option: http://www.example.com/Path
Receipt-delivery-option syntax allows return-url to use some schemes
other than HTTP using the POST method.
The "receipt-delivery-option: return-url" string indicates the URL to
use for an asynchronous MDN. This header is NOT present if the
receipt is to be synchronous. The email value in Disposition-
notification-to is not used in this specification because it was
limited to RFC 2822 addresses; the extension header "Receipt-
delivery-option" has been introduced to provide a URL for the MDN
return by several transfer options.
The receipt-delivery-option's value MUST be a URL indicating the
delivery transport destination for the receipt.
An example request for an asynchronous MDN via an HTTP transport:
Receipt-delivery-option: http://www.example.com
An example request for an asynchronous MDN via an HTTP/S transport:
Receipt-delivery-option: https://www.example.com
An example request for an asynchronous MDN via an SMTP transport:
Receipt-delivery-option: mailto:as2@example.com
For more information on requesting SMTP MDNs, refer to RFC 3335 [4].
Finally, the header, Disposition-notification-options, identifies
characteristics of message disposition notification as in [5]. The
most important of these options is for indicating the signing options
for the MDN, as in the following example:
Disposition-notification-options:
signed-receipt-protocol=optional,pkcs7-signature;
signed-receipt-micalg=optional,sha1,md5
For signing options, consider the disposition-notification-options
syntax:
Disposition-notification-options =
"Disposition-Notification-Options" ":"
disposition-notification-parameters
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where
disposition-notification-parameters =
parameter *(";" parameter)
where
parameter = attribute "=" importance ", " 1#value"
where
importance = "required" | "optional"
So the Disposition-notification-options string could be:
signed-receipt-protocol=optional,<protocol symbol>;
signed-receipt-micalg=optional,<micalg1>,<micalg2>,...;
The currently used value for <protocol symbol> is "pkcs7-signature"
for the S/MIME detached signature format.
The currently supported values for MIC algorithm <micalg> values are:
Algorithm Value Used
--------- -------
SHA-1 sha1
MD5 md5
The semantics of the "signed-receipt-protocol" and the "signed-
receipt-micalg" parameters are as follows:
1. The "signed-receipt-protocol" parameter is used to request a
signed receipt from the recipient trading partner. The "signed-
receipt-protocol" parameter also specifies the format in which the
signed receipt SHOULD be returned to the requester.
The "signed-receipt-micalg" parameter is a list of MIC algorithms
preferred by the requester for use in signing the returned
receipt. The list of MIC algorithms SHOULD be honored by the
recipient from left to right.
Both the "signed-receipt-protocol" and the "signed- receipt-
micalg" option parameters are REQUIRED when requesting a signed
receipt.
The lack of the presence of the "Receipt-Delivery-Option"
indicates that a receipt is synchronous in nature. The presence
of the "Receipt-Delivery-Option: return-url" indicates that an
asynchronous receipt is requested and SHOULD be sent to the
"return-url".
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2. The "importance" attribute of "Optional" is defined in RFC 3798,
Section 2.2, and has the following meaning:
Parameters with an importance of "Optional" permit a UA that does
not understand the particular options parameter to still generate
an MDN in response to a request for a MDN.
A UA that does not understand the "signed-receipt-protocol"
parameter or the "signed-receipt-micalg" will obviously not return
a signed receipt.
The importance of "Optional" is used for the signed receipt
parameters because it is RECOMMENDED that an MDN be returned to
the requesting trading partner even if the recipient could not
sign it.
The returned MDN will contain information on the disposition of
the message and on why the MDN could not be signed. See the
Disposition field in Section 7.5 for more information.
Within an EDI trading relationship, if a signed receipt is
expected and is not returned, then the validity of the transaction
is up to the trading partners to resolve.
In general, if a signed receipt is required in the trading
relationship and is not received, the transaction will likely not
be considered valid.
7.3.1. Signed Receipt Considerations
The method used to request a receipt or a signed receipt is defined
in RFC 3798, "An Extensible Message Format for Message Disposition
Notifications".
The "rules" are as follows:
1. When a receipt is requested, explicitly specifying that the
receipt be signed, then the receipt MUST be returned with a
signature.
2. When a receipt is requested, explicitly specifying that the
receipt be signed, but the recipient cannot support either the
requested protocol format or the requested MIC algorithms, then
either a signed or unsigned receipt SHOULD be returned.
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3. When a signature is not explicitly requested, or if the signed
receipt request parameter is not recognized by the UA, then no
receipt, an unsigned receipt, or a signed receipt MAY be returned
by the recipient.
NOTE: For Internet EDI, it is RECOMMENDED that when a signature is
not explicitly requested, or if parameters are not recognized, the UA
send back, at a minimum, an unsigned receipt. If, however, a signed
receipt was always returned as a policy, whether requested or not,
then any false unsigned receipts can be repudiated.
When a request for a signed receipt is made, but there is an error in
processing the contents of the message, a signed receipt MUST still
be returned. The request for a signed receipt SHALL still be
honored, though the transaction itself may not be valid. The reason
why the contents could not be processed MUST be set in the
"disposition-field".
When a signed receipt request is made, the "Received-content-MIC"
MUST always be returned to the requester (except when corruption
prevents computation of the digest in accordance with the following
specification). The "Received-content-MIC" MUST be calculated as
follows:
o For any signed messages, the MIC to be returned is calculated
on the RFC1767/RFC3023 MIME header and content.
Canonicalization on the MIME headers MUST be performed before
the MIC is calculated, since the sender requesting the signed
receipt was also REQUIRED to canonicalize.
o For encrypted, unsigned messages, the MIC to be returned is
calculated on the decrypted RFC 1767/RFC3023 MIME header and
content. The content after decryption MUST be canonicalized
before the MIC is calculated.
o For unsigned, unencrypted messages, the MIC MUST be calculated
over the message contents without the MIME or any other RFC2822 headers, since these are sometimes altered or reordered by
Mail Transport Agents (MTAs).
7.4. MDN Format and Values
This section defines the format of the AS2 Message Disposition
Notification (AS2-MDN).
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of "multipart/report" and parameters indicating the report-type, and
the value of the outermost multipart boundary.
When the AS2-MDN is signed, the transfer-layer ("outermost") entity-
headers of the AS2-MDN contain a content-type header that specifies a
content-type of "multipart/signed" and parameters indicating the
algorithm used to compute the message digest, the signature-
formatting protocol (e.g., pkcs7-signature), and the value of the
outermost multipart boundary. The first part of the MIME
multipart/signed message is an embedded MIME multipart/report of type
"disposition-notification". The second part of the multipart/signed
message contains a MIME application/pkcs7-signature message.
The first part of the MIME multipart/report is a "human-readable"
portion that contains a general description of the message
disposition. The second part of the MIME multipart/report is a
"machine-readable" portion that is defined as:
AS2-disposition-notification-content =
[ reporting-ua-field CRLF ]
[ mdn-gateway-field CRLF ]
final-recipient-field CRLF
[ original-message-id-field CRLF ]
AS2-disposition-field CRLF
*( failure-field CRLF )
*( error-field CRLF )
*( warning-field CRLF )
*( extension-field CRLF )
[ AS2-received-content-MIC-field CRLF ]
7.4.3. AS2-MDN Fields
The rules for constructing the AS2-disposition-notification content
are identical to the disposition-notification-content rules provided
in Section 7 of RFC 3798 [5], except that the RFC 3798 disposition-
field has been replaced with the AS2-disposition-field and that the
AS2-received-content-MIC field has been added. The differences
between the RFC 3798 disposition-field and the AS2-disposition-field
are described below. Where there are differences between this
document and RFC 3798, those entity names have been changed by pre-
pending "AS2-". Entities that do not differ from RFC 3798 are not
necessarily further defined in this document; refer to RFC 3798,
Section 7, "Collected Grammar", for the original grammar.
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The "Received-content-MIC" extension field is set when the integrity
of the received message is verified. The MIC is the base64-encoded
message-digest computed over the received message with a hash
function. This field is required for signed receipts but optional
for unsigned receipts. For details defining the specific content
over which the message digest is to be computed, see Section 7.3.1 of
this document.
For signed messages, the algorithm used to calculate the MIC MUST be
the same as that used on the message that was signed. If the message
is not signed, then the SHA-1 algorithm SHOULD be used. This field
is set only when the contents of the message are processed
successfully. This field is used in conjunction with the recipient's
signature on the MDN so that the sender can verify non-repudiation of
receipt.
AS2-MDN field names (e.g., "Disposition:", "Final-Recipient:") are
case insensitive (cf. RFC 3798, Section 3.1.1). AS2-MDN action-
modes, sending-modes, AS2-disposition-types, and AS2-disposition-
modifier values, which are defined above, and user-supplied *( TEXT )
values are also case insensitive. AS2 implementations MUST NOT make
assumptions regarding the values supplied for AS2-MDN-warning-
description or AS2-MDN-failure-description, or for the values of any
(optional) error, warning, or failure fields.
7.4.4. Additional AS2-MDN Programming Notes
o Unlike SMTP, for HTTP transactions, Original-Recipient and Final-
Recipient SHOULD not be different. The value in Original-
Message-ID SHOULD match the original Message-ID header value.
o Refer to RFC 3798 for the formatting of the MDN, except for the
specific deviations mentioned above.
o Refer to RFC 3462 and RFC 3798 for the formatting of the content-
type entity-headers for the MDN.
o Use an action-mode of "automatic-action" when the disposition
described by the disposition type was a result of an automatic
action rather than that of an explicit instruction by the user for
this message.
o Use an action-mode of "manual-action" when the disposition
described by the disposition type was a result of an explicit
instruction by the user rather than some sort of automatically
performed action.
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o Use a sending-mode of "MDN-sent-automatically" when the MDN is
sent because the UA had previously been configured to do so.
o Use a sending-mode of "MDN-sent-manually" when the user explicitly
gave permission for this particular MDN to be sent.
o The sending-mode "MDN-sent-manually" is meaningful ONLY with
"manual-action", not with "automatic-action".
o The "failed" disposition type MUST NOT be used for the situation
in which there is some problem in processing the message other
than interpreting the request for an MDN. The "processed" or
other disposition type with appropriate disposition modifiers is
to be used in such situations.
7.5. Disposition Mode, Type, and Modifier7.5.1. Disposition Mode Overview
This section provides a brief overview of how "processed", "error",
"failure", and "warning" are used.
7.5.2. Successful Processing Status Indication
When the request for a receipt or signed receipt, and the received
message contents are successfully processed by the receiving EDI UA,
a receipt or MDN SHOULD be returned with the disposition-type set to
"processed". When the MDN is sent automatically by the EDI UA, and
there is no explicit way for a user to control the sending of the
MDN, then the first part of the "disposition-mode" SHOULD be set to
"automatic-action". When the MDN is being sent under user-
configurable control, then the first part of the "disposition-mode"
SHOULD be set to "manual-action". Since a request for a signed
receipt should always be honored, the user MUST not be allowed to
configure the UA not to send a signed receipt when the sender
requests one.
The second part of the disposition-mode is set to "MDN-sent-manually"
if the user gave explicit permission for the MDN to be sent. Again,
the user MUST not be allowed to explicitly refuse to send a signed
receipt when the sender requests one. The second part of the
"disposition-mode" is set to "MDN-sent-automatically" whenever the
EDI UA sends the MDN automatically, regardless of whether the sending
was under the control of a user, administrator, or software.
Because EDI content is generally handled automatically by the EDI UA,
a request for a receipt or signed receipt will generally return the
following in the "disposition-field":
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Disposition: automatic-action/MDN-sent-automatically; processed
Note that this specification does not restrict the use of the
"disposition-mode" just to automatic actions. Manual actions are
valid as long as it is kept in mind that a request for a signed
receipt MUST be honored.
7.5.3. Unsuccessful Processed Content
The request for a signed receipt requires the use of two
"disposition-notification-options", which specify the protocol format
of the returned signed receipt, and the MIC algorithm used to
calculate the MIC over the message contents. The "disposition-field"
values that should be used if the message content is being rejected
or ignored (for instance, if the EDI UA determines that a signed
receipt cannot be returned because it does not support the requested
protocol format, the EDI UA chooses not to process the message
contents itself) MUST be specified in the MDN "disposition-field" as
follows:
Disposition: "disposition-mode"; failed/Failure:
unsupported format
The "failed" AS2-disposition-type MUST be used when a failure occurs
that prevents the proper generation of an MDN. For example, this
disposition-type would apply if the sender of the message requested
the application of an unsupported message-integrity-check (MIC)
algorithm.
The "failure:" AS2-disposition-modifier-extension SHOULD be used with
an implementation-defined description of the failure. Further
information about the failure may be contained in a failure-field.
The syntax of the "failed" disposition-type is general, allowing the
sending of any textual information along with the "failed"
disposition-type. Implementations MUST support any printable textual
characters after the Failure disposition-type. For use in Internet
EDI, the following "failed" values are pre-defined and MUST be
supported:
"Failure: unsupported format"
"Failure: unsupported MIC-algorithms"
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 20057.5.4. Unsuccessful Non-Content Processing
When errors occur in processing the received message (other than
content), the "disposition-field" MUST be set to the "processed"
value for disposition-type and the "error" value for disposition-
modifier.
The "error" AS2-disposition-modifier with the "processed"
disposition-type MUST be used to indicate that an error of some sort
occurred that prevented successful processing of the message.
Further information may be contained in an error-field.
An "error:" AS2-disposition-modifier-extension SHOULD be used to
combine the indication of an error with a predefined description of a
specific, well-known error. Further information about the error may
be contained in an error field.
For internet EDI use, the following "error" AS2-disposition-modifier
values are defined:
o "Error: decryption-failed" - the receiver could not
decrypt the message
contents.
o "Error: authentication-failed" - the receiver could not
authenticate the sender.
o "Error: integrity-check-failed" - the receiver could not
verify content integrity.
o "Error: unexpected-processing-error" - a catch-all for any
additional processing
errors.
An example of how the "disposition-field" would look when errors
other than those in content processing are detected is as follows:
Disposition: "disposition-mode"; processed/Error:
decryption-failed
7.5.5. Processing Warnings
Situations arise in EDI when, even if a trading partner cannot be
authenticated correctly, the trading partners still agree to continue
processing the EDI transactions. Transaction reconciliation is done
between the trading partners at a later time. In the content
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
processing warning situations as described above, the "disposition-
field" MUST be set to the "processed" disposition-type value, and the
"warning" to the "disposition-modifier" value.
The "warning" AS2-disposition-modifier MUST be used with the
"processed" disposition-type to indicate that the message was
successfully processed but that an exceptional condition occurred.
Further information may be contained in a warning-field.
A "warning:" AS2-disposition-modifier-extension SHOULD be used to
combine the indication of a warning with an implementation-defined
description of the warning. Further information about the warning
may be contained in a warning-field.
For use in Internet EDI, the following "warning"
disposition-modifier-extension value is defined:
"Warning: authentication-failed, processing continued"
An example of how the "disposition-field" would look when warning
other than those for content processing are detected is as follows:
Example:
Disposition: "disposition-mode"; processed/Warning:
authentication-failed, processing continued
7.5.6. Backward Compatibility with Disposition Type, Modifier, and Extension
The following set of examples represents typical constructions of the
Disposition field that have been in use by AS2 implementations. This
is NOT an exhaustive list of possible constructions. However, AS2
implementations MUST accept constructions of this type to be backward
compatible with earlier AS2 versions.
Disposition: automatic-action/MDN-sent-automatically; processed
Disposition: automatic-action/MDN-sent-automatically;
processed/error: authentication-failed
Disposition: automatic-action/MDN-sent-automatically;
processed/warning: duplicate-document
Disposition: automatic-action/MDN-sent-automatically;
failed/failure: sender-equals-receiver
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The following set of examples represents allowable constructions of
the Disposition field that combine the historic constructions above
with optional RFC 3798 error, warning, and failure fields. AS2
implementations MAY produce these constructions. However, AS2
servers are not required to recognize or process optional error,
warning, or failure fields at this time. Note that the use of the
multiple error fields in the second example below provides for the
indication of multiple error conditions.
Disposition: automatic-action/MDN-sent-automatically; processed
Disposition: automatic-action/MDN-sent-automatically;
processed/error: decryption-failed
Error: The signature did not decrypt into a valid PKCS#1
Type-2 block.
Error: The length of the decrypted key does not equal the
octet length of the modulus.
Disposition: automatic-action/MDN-sent-automatically;
processed/warning: duplicate-document
Warning: An identical message already exists at the
destination server.
Disposition: automatic-action/MDN-sent-automatically;
failed/failure: sender-equals-receiver
Failure: The AS2-To name is identical to the AS2-From name.
The following set of examples represents allowable constructions of
the Disposition field that employ pure RFC 3798 Disposition-modifiers
with optional error, warning, and failure fields. These examples are
provided as informational only. These constructions are not
guaranteed to be backward compatible with AS2 implementations prior
to version 1.1.
Disposition: automatic-action/MDN-sent-automatically; processed
Disposition: automatic-action/MDN-sent-automatically;
processed/error
Error: authentication-failed
Error: The signature did not decrypt into a valid PKCS#1 Type-2
block.
Error: The length of the decrypted key does not equal the
octet length of the modulus.
Disposition: automatic-action/MDN-sent-automatically;
processed/warning
Warning: duplicate-document
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Disposition: automatic-action/MDN-sent-automatically; failed
Failure: sender-equals-receiver
7.6. Receipt Reply Considerations in an HTTP POST
The details of the response to the POST command vary depending upon
whether a receipt has been requested.
With no extended header requesting a receipt, and with no errors
accessing the request-URI specified processing, the status line in
the Response to the POST request SHOULD be in the 200 range. Status
codes in the 200 range SHOULD also be used when an entity is returned
(a signed receipt in a multipart/signed content type or an unsigned
receipt in a multipart/report). Even when the disposition of the
data was an error condition at the authentication, decryption or
other higher level, the HTTP status code SHOULD indicate success at
the HTTP level.
The HTTP server-side application may respond with an unsolicited
multipart/report as a message body that the HTTP client might not
have solicited, but the client may discard this. Applications SHOULD
avoid emitting unsolicited receipt replies because bandwidth or
processing limitations might have led administrators to suspend
asking for acknowledgements.
Message Disposition Notifications, when used in the HTTP reply
context, will closely parallel a SMTP MDN. For example, the
disposition field is a required element in the machine-readable
second part of a multipart/report for a MDN. The final-recipient-
field ([5], Section 3.1) value SHOULD be derived from the entity
headers of the request.
In an MDN, the first part of the multipart/report (the human-readable
part) SHOULD include items such as the subject, the date, and other
information when those fields are present in entity header fields
following the POST request. An application MUST report the Message-
ID of the request in the second part of the multipart/report (the
machine-readable part). Also, an MDN SHOULD have its own unique
Message-ID HTTP header. The HTTP reply SHOULD normally omit the
third optional part of the multipart/report (used to return the
original message or its headers in the SMTP context).
8. Public Key Certificate Handling
In the near term, the exchange of public keys and certification of
these keys MUST be handled as part of the process of establishing a
trading partnership. The UA and/or EDI application interface must
maintain a database of public keys used for encryption or signatures,
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
in addition to the mapping between the EDI trading partner ID and the
RFC 2822 [9] email address and HTTP URL/URI. The procedures for
establishing a trading partnership and configuring the secure EDI
messaging system might vary among trading partners and software
packages.
X.509 certificates are REQUIRED. It is RECOMMENDED that trading
partners self-certify each other if an agreed-upon certification
authority is not used. This applicability statement does NOT require
the use of a certification authority. The use of a certification
authority is therefore OPTIONAL. Certificates may be self-signed.
It is RECOMMENDED that when trading partners are using S/MIME they
also exchange public key certificates, considering advice provided in
[12].
The message formats useful for certificate exchange are found in [7]
and [13].
In the long term, additional standards may be developed to simplify
the process of establishing a trading partnership, including the
third-party authentication of trading partners, as well as the
attributes of the trading relationship.
9. Security Considerations
This entire document is concerned with secure transport of business
to business data, and it considers both data confidentiality and
authentication issues.
Extracted from RFC 3851 [7]:
40-bit encryption is considered weak by most cryptographers. Using
weak cryptography in S/MIME offers little actual security over
sending plaintext. However, other features of S/MIME, such as the
specification of Triple DES and the ability to announce stronger
cryptographic capabilities to parties with whom you communicate,
allow senders to create messages that use strong encryption. Using
weak cryptography is never recommended unless the only alternative is
no cryptography. When feasible, sending and receiving agents SHOULD
inform senders and recipients of the relative cryptographic strength
of messages.
Extracted from RFC 3850 [12]:
When processing certificates, there are many situations where the
processing might fail. Because the processing may be done by a user
agent, a security gateway, or other program, there is no single way
to handle such failures. Just because the methods to handle the
failures have not been listed, however, the reader should not assume
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
that they are not important. The opposite is true: if a certificate
is not provably valid and associated with the message, the processing
software should take immediate and noticeable steps to inform the end
user about it.
Some of the many situations in which signature and certificate
checking might fail include the following:
o No certificate chain leads to a trusted CA.
o No ability to check the Certificate Revocation List (CRL) for a
certificate.
o An invalid CRL was received.
o The CRL being checked is expired.
o The certificate is expired.
o The certificate has been revoked.
There are certainly other instances where a certificate may be
invalid, and it is the responsibility of the processing software to
check them all thoroughly, and to decide what to do if the check
fails. See RFC 3280 for additional information on certificate path
validation.
The following are additional security considerations to those listed
in [7] and [12].
9.1. NRR Cautions
This specification seeks to provide multiple mechanisms that can be
combined in accordance with local policies to achieve a wide range of
security needs as determined by threat and risk analyses of the
business peers. It is required that all these mechanisms be
implemented by AS2 software so that the software has capabilities
that promote strong interoperability, no matter what policies are
adopted.
One strong cluster of mechanisms (the secure transmission loop) can
provide good support for meeting the evidentiary needs of non-
repudiation of receipt by the original sender and by a third party
supplied with all stated evidence. However, this specification does
not itself define non-repudiation of receipt nor enumerate its
essential properties because NRR is a business analysis and/or legal
requirement, and not relevantly defined by a technical applicability
statement.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Some analyses observe that non-repudiation of receipt presupposes
that non-repudiation of the sender of the original message is
obtained, and further that non-repudiation should be implemented by
means of digital signature on the original message. To satisfy
strict NRR evidence, authentication and integrity MUST be provided by
some mechanism, and the RECOMMENDED mechanism is digital signatures
on both the original message and the receipt message.
Given that this specification has selected several mechanisms that
can be combined in several ways, it is important to realize that if a
digital signature is omitted from the original message, in order to
satisfy the preceding analysis of NRR requirements, some
authentication mechanism MUST accompany the request for a signed
receipt and its included Received-content-MIC value. This
authentication might come from using client-side SSL, authentication
via IPsec, or HTTP authentication (while using SSL). In any case,
records of the message content, its security basis, and the digest
value need to be retained for the NRR process.
Therefore, if NRR is one of the goals of the policy that is adopted,
by using the mechanisms of the secure transmission loop mentioned
above and by retaining appropriate records of authentication at the
original message sender site, strong evidentiary requirements
proposed for NRR can be fulfilled.
Other ways of proceeding may fall short of fulfilling the most
stringent sets of evidence required for NRR to obtain, but may
nevertheless be part of a commercial trading agreement and, as such,
are good enough for the parties involved. However, if MDNs are
returned unsigned, evidentiary requirements for NRR are weak; some
authentication of the identity of the receiver is needed.
9.2. HTTPS Remark
The following certificate types MUST be supported for SSL server-side
certificates:
o with URL in the Distinguished Name Common Name attribute
o without URL in the Distinguished Name Common Name attribute
o self-signed (self-issued)
o certification authority certified
The URL, which matches the source server identity, SHOULD be carried
in the certificate. However, it is not required that DNS checks or
reverse lookups to vouch for the accuracy of the URL or server value.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Because server-side certificates are exchanged, and also trust is
established during the configuration of the trading partner
relationship, runtime checks are not required by implementations of
this specification.
The complete certification chain MUST be included in all
certificates. All certificate verifications MUST "chain to root" or
to an accepted trust anchor. Additionally, the certificate hash
SHOULD match the hash recomputed by the receiver.
9.3. Replay Remark
Because business data documents normally contain transaction ids,
replays (such as resends of not-yet-acknowledged messages) are
discarded as part of the normal process of duplicate detection.
Detection of duplicates by Message-Id or by business transaction
identifiers is recommended.
10. IANA ConsiderationsRFC 3335 registered two Disposition-Notification-Options parameters
Parameter-name: signed-receipt-protocol
Parameter-name: signed-receipt-micalg
that are also used by this specification (see Section 7.3).
RFC 3335 also registered on MDN Extension field name
Extension field name: Received-content-MIC
that is also used by this specification (see Section 7.4.3).
Registration of the above is therefore NOT needed.
10.1. Registration
This specification defines an extension to the Message Disposition
Notification (MDN) protocol for a disposition-modifier in the
Disposition field of a body of content-type "message/disposition-
notification".
10.1.1. Disposition Modifier 'warning'
Parameter-name: warning
Semantics: See Sections 7.4.3 and 7.5.5 of this document.
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
Notes:
1. The lines proceeded with "&" are what the signature is calculated
over.
2. For details on how to prepare the multipart/signed with protocol =
"application/pkcs7-signature", see the "S/MIME Message
Specification, PKCS Security Services for MIME".)
3. Note that the textual first body part of the multipart/report can
be used to include a more detailed explanation of the error
conditions reported by the disposition headers. The first body
part of the multipart/report, when used in this way, allows a
person to better diagnose a problem in detail.
4. As specified by RFC 3462 [8], returning the original or portions
of the original message in the third body part of the
multipart/report is not required. This is an optional body part.
However, it is RECOMMENDED that this body part be omitted or left
blank.
A.3. Signed, Encrypted Message Requesting a Signed, Asynchronous
Receipt
Message-ID: <#as2_company#01#a4260as2_companyout#>
Date: Thu, 19 Dec 2002 15:04:18 GMT
From: me@example.com
Subject: Async MDN request
Mime-Version: 1.0
Content-Type: application/pkcs7-mime;
smime-type=enveloped-data; name=smime.p7m
Content-Transfer-Encoding: binary
Content-Disposition: attachment; filename=smime.p7m
Recipient-Address: 10.240.1.2//
Disposition-Notification-To:
http://10.240.1.2:8201/exchange/as2_company
Disposition-Notification-Options: signed-receipt-protocol=optional,
pkcs7-signature; signed-receipt-micalg=optional,sha1
Receipt-Delivery-Option:
http://10.240.1.2:8201/exchange/as2_company
AS2-From: as2_company
AS2-To: "AS2 Test"
AS2-Version: 1.1
Host: 10.240.1.2:8101
Connection: close
Content-Length: 3428
[omitted binary encrypted data]
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RFC 4130 AS2 for Business Data Interchange Using HTTP July 2005
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